Black holes belch up after years of tearing apart stars, and we don’t know why

For a few hours after a star collides with a supermassive black hole, some of the brightest light in the universe is produced.

The subsequent radio flash is thought to subside within weeks or months of the collision. It turns out we might have been a little impatient to turn our gaze elsewhere.

An international team of astrophysicists has witnessed radio waves blasting from the material surrounding a variety of supermassive black holes hundreds of days after tearing apart a star, suggesting that many collisions could be responsible for a serious case of cosmic dysphoria.

“As many as half of all black holes tear apart star material years after the initial event.” He says Astrophysicist and lead author Yvette Sindis. “No one expected this, and we don’t really understand why this might be!”

Using observations from three radio telescopes – the Very Large Array in the United States, MEARKAT in South Africa, and the Australian Telescope’s Compact Array – the team collected long-term data on 24 black holes.

Of those twenty giants, ten sat quietly for about 500 to 2,000 after enjoying their excellent lunch only to set off a burst of radio waves.

Their findings have not yet been peer-reviewed, but are currently available on the preprint server arxiv.org for anyone to dig into.

When a star gets too close to a black hole, the intense force of gravity causes it to elongate into a spaghetti shape. Within hours, the star is torn to pieces in what is called a “tidal disturbance event.” This results in one of the brightest light flares in the universe.

About 20 to 30 percent of these tidal disturbance events will produce a burst of radio waves in the early stages. Only about 100 of these tidal disturbance events have been seen since the first one was recorded in the 1990s.

Once they notice this bright light, researchers usually move on to other things, since “radio telescope time is precious,” Sindis explains in a Reddit thread. “For example, why do we go to the site of the explosion years after it happened?”

A discovery made last year appears to have turned this logic on its head. Cindis and her team found that a black hole 20 million times larger than the Sun was emitting jets of radio waves about three years after tearing apart a star, which was “completely wild,” Cindis says.

The team named this black hole “Jetty McJetface,” or “Jetty” for short.

“Getty was just one of 24 tidal disturbance events we were studying…so what were the rest of them doing?!” Cindis says.

Contrary to popular belief, black holes do not suck up stars like a vacuum cleaner. They consume stars like the Cookie Monster eats cookies: by creating chaos.

Very little of the stellar material that collides with a black hole actually passes through the event horizon (behind which the gravitational force is so great that no light can escape).

About half of the stellar material is expelled outward into the galaxy, while the other half joins the debris orbiting the black hole, called the accretion disk.

There are two possible reasons why the remains of stars orbiting black holes might begin emitting radio waves years after the collision, the researchers wrote.

One possibility is that it takes a long time for the debris orbiting the black hole to settle into a stable orbit.

The alternative is that the debris is weakly bound to the supermassive black hole and forms a spherical shell, which “must cool and shrink radically to form an accretion disk,” the authors write.

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“If accretion onto the supermassive black hole provides energy to the envelope, complete contraction of the envelope could be delayed by up to ~700 days, in agreement with the time scales of outflows measured in this work.

“Thus, we see that disk formation could be delayed by hundreds to thousands of days, providing an alternative explanation for delayed radio bursts from the supermassive black hole.”

This paper appeared as a preprint on arXiv.org.